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Dissolution of Potash Ore in Solutions of High Sodium Chloride Concentration.

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Dissolution of Potash Ore in Solutions of High
Sodium Chloride Concentration
RG. Gilliesl, D.N. Madge* and CA. Shook*
Depatfmenf of Chemical Engineering, University of Saskatchewan,
Saskatoon S7N OW, CANADA
An experimental investigation of the dissolution of KCI-containing ore in brines of high
NaCl content has been conducted. The dissolving su@ace was vertical and motion of the
solution was due only tofree convection. The effects of ore composition and brine NaCl
concentrationwere studied for unsaturated brines using both reconstituted ore and raw
ore slabs (11J cm high). These dissolution rates were consistent with previous free
convection heat and maps transfer results, when the interfacial solution was assumed to
be saturated with respect to both KCl and NaCl (the invariantpoint composition).
Dissolution by brines saturated with respect to NaCl was investigated with respect to
the effects of crystal size in the slabs, solution KCl concentration, temperature and time.
Dissolution rates were much lower in these cases and appeared to be controlled by a
surface reaction. The mechanism of dissolution appears to involve penetration of the
slab bv the dissolvinz solution.
Introduction
Potash has been mined in Saskatchewan since 1958 and at present it is the most
important mineral produced in the province. Since vast quantities of NaCl-rich/KCllean railings have been left on the surface, the possible dissolution of these deposits
is a matter of concern. With growing interest in solution mining (one of the original
shaft mines has been converted to a solution mine) and with recent concern about
migration of aqueous solutions underground, the dynamic behaviour of the system
NaCl-KCl-%O is of interest.
The present investigation was undertaken to examine ore dissolution rates in
aqueous solutions containing KCl and NaCl. The region of greatest interest was that
of relatively high NaCl concentrations. As in a previous investigation (Husband and
Oszahin, 1967) the variability and insoluble content of the natural ore required
reconstituted ores to be fabricated in an attempt to obtain reproducible experimental
conditions.
Dissolution rates were to be determined using equipment of laboratory scale. In
view of the complexity introduced by the presence of two species dissolving
simultaneously,the experimentalconditions were selected to provide a regime whose
hydrodynamic conditions are relatively well understood. The particular situation
was free convective dissolution of vertical slabs immersed in a large quantity of the
solution.
~
Saskatchewan Research Council, Sashtoon, CANADA
Kilborn Western Inc., Sashtoon, CANADA
* Authorfor correspondence.
218
Dissolution of Potash Ore in Solutions ofHigh Sodium chloride Concentratwn
Since brines saturated with respect to NaCl can be produced in a wide variety of
physical situations which occur above and below ground, the experimental program
was also designed to examine the behaviour of ores in contact with a brine of this
type. Both natural and reconstituted ore slabs were used in this phase of the
investigation.
(a) Free Convective Dissolution of Vertical Surfaces
Molecular diffusion of a single solute (component 1) is described by Fick's Law:
where u is the component of the mean velocity of the solution in the direction of N.
For transfer in the y direction:
Component 2 is the other constituent of the binary mixture.
For liquids, because the mass density is nearly constant, an appropriate mass
transfer rate equation for engineering design is:
where k, is the mass transfer coefficient which depends, among other factors, upon
the diffusivity and the hydrodynamic conditions in the vicinity of the dissolving
surface.
At low rates of mass transfer, single component mass transfer and heat transfer
are strongly analogous; the relationship is implicit in the conservation laws for mass,
momentum and heat, and in the physical laws governing the diffusion of heat and
mass by molecular motion. In laminar free convective heat m s f e r from a vertical
plate, it can be shown that:
Nu = f (Gr, Pr)
(4)
where the Nusselt number, Nu = h uk;L is the length of the surface; k is the thermal
conductivity; Pr is the fluid Prandtl number, FV = pC@; C, is the heat capacity. The
Grashof number (Gr) is the ratio of buoyancy forces to inertial forces:
The mass transfer relationship which is analogous to Equation (4) is:
Sh" = f (Gr, Sc)
where the Schmidt number, Sc = NpD,; and the Sherwood number, Sh" = Isp L/D1.
The product (Gr Sc) is the Rayleigh number for mass transfer. The coefficientlip is
the limiting value of the mass transfer coefficient at very low rates. For highly
219
R.G. Gillies, DN.Madge and CA. Shook
soluble substances, the relationship between these mass transfer coefficients is given
by:
Although the difference between k and kqo is recognized in forced convection
mass transfer, earlier free convective dissolution studies often ignored the difference.
Experimental data have been presented in the form of Sherwood numbers defined as:
and plotted in terms of the Rayleigh number (Gr Sc). These resdts are usually
compared to those obtained in heat transfer, for which an accepted correlation for
laminar boundary layers is:
Nu = 0.667 (Gr Pr)o-25
(6)
Equation (6) can be derived by considering the velocity and thermal boundary
layers in free convection. If the boundary layer becomes turbulent, then the
correlation of Churchdl and Chu (1975) is appropriate:
Nu0q5 = 0.825 + 0.387 (Gr Pr)'I6/[1
+ (0.492/Pr)9/16]8''27
(7)
100000
10000
--
-
Churchill h Chu
Husband Experiment
-
1000
100
I~
I I1111
10
1
;
II
11
Ills
Id
m
GrSc
1 1
1
T IS
111-
18
10
Figure 1 . Comparison of the experimental Shenvood numbers of Husband and
Shook (invert sugar dissolution) with the correlation of Churchill and Chu.
If the mass transfer process is one of diffusion controlled by free convection at
the surface, then Equation (7) can be used to predict mass transfer coefficients with
Nu = Sh and Pr = Sc, in both laminar and turbulent regions. The applicability of the
Churchill and Chu (1975) correlation to free convection mass transfer is shown in
Figure 1, which compares the experimental results of Husband and Shook (1970)
220
Dissolauion of Potash Ore in SolutiorrS @High Sodium Chloride Concenlratwn
with the correlation. Husband's experiments used invert sugar slabs for which Sc =
2.89 x 106 and xls = 0.23, and agreement with the correlation is seen to be excellent.
In these experiments (Husband, 1969), slabs with L values ranging between 7.6 cm
and 6.1 m were used and it was noted that the precision of the measurements
improved as L increased.
The transition from a laminar to a arrbulent boundary layer occurs at a value of
(Gr Sc) near lo9 for low Pr fluids, while Husband (1969) found that it occurred at
(Gr Sc) near 1013with the invert sugar slabs.
If a chemical reaction occurs at the surface as part of the dissolution process,
mass transfer coefficients predicted by Equation (7) will be higher than the
experimental values. Evidence for a surface reaction in forced convective dissolution
of pure KCl into NaCl solutions and pure NaCl into KC1 solutions was given by
Simon (1981).
(b) Potash Ore Dissolution by Unsaturated Brines
When two salts are present, Equation (1) can be applied to each component at any
point on the dissolving surface. However, Equation (2) requires modification (eg.
Fujita and Gosting, 1960):
J, = -D,
acl/ay - ~ 1acday
3
and
where components 1 and 3 are identifed as NaCl and KCl respectively. The crossterm diffusion coefficients D13 and D31 are approximately 15 to 20%of D, and D3.
If Equation (3) is to be used to calculate dissolution rates, the surface
compositions (xis, x3J are required and these will vary from point to point. Figure 2
shows the loci of isothermal saturation compositions in the form of mass ratios of
NaCkwater and KC1:water in the region of high NaCl concentrations. Curve AB
shows solutions saturated with respect to NaCI; the negative slope indicates the
common-ion (Cl-) effect. Curve BC applies to solutions saturated with respect to
KCl. At the junction (B), the invariant point, the solution is saturatedwith respect to
both salts.
In the coordinate system of Figure 2, a dry mixture of NaCl and KCl would be
represented by a point at infinity, lying on a line passing through the origin, with a
slope determined by the ratio of NaCl to KCl. In interpreting their dissolution rate
measurements, Husband and Oszahin (1967) appear to have defined the surface
compositions (xh and x 3 by the intersection of this line with curves AB and BC. This
is certainly a plausible engineering simplifying assumption but it is not the only one.
For example, when the total perimeter of the junctions between NaCl and KCl crystals
per unit area of dissolving &ace is high, it would be equally appropriate to assume
that the mean d a c e concenmtionfor the slab as a whole was that of point B.
221
R.G. Gillies,DN.Madge and C A . Shook
0.4
I
*
Q)
CP
Y
0.3
1
0
u 0.2
7
CP
Y
0.1
I
0.0
1
0.j
I
0.2
I
0.3
kg KCl/kg water
Figure 2 . Phase diagram for the system NaCl-KCl-water at 29OC. (The solid
points depict the solutions used in the short-term experiments).
Husband and Oszahin (1967) reported good agreement between their three
measured initial dissolution rates, as determined with reconstituted ore slabs
containing approximately 50% KCl, and the predictions made with Equation (6). In
their experiments, L was approximately 21 cm. In order to obtain this good
agreement, it was necessary to extrapolate measured rates to overcome the effects of
turbulence and roughness which developed on the surface.
(c) Dissolution by Saturated Brines
According to the mechanism implicit in the method of Husband and Oszahin (1967).
brine in contact with ore will eventually become saturated with respect to one of the
salts. Once saturation occurs, according to this view, dissolution should cease. This
situation was examined experimentally by Taylor et al. (1967). Using ore samples
exposed to brine at their lower surfaces, deposition ('blinding') of NaCl on the
surface during KCl dissolution was observed when the solution was sahrrslted with
respect to NaC1. Simon (1981) found that deposition of KC1 on an NaCl crystal
occurred very rapidly, when the solution KC1 concentration was high, even when the
solution was undersaturated in both salts. Similarly, NaCl was deposited very
rapidly on a KCl crystal from a solution of high but undersaturated NaCl
concentration. These observations suggest that dissolution in brines of high salinity
could decrease rapidly with time, or cease altogether.
When an ore is placed in contact with a saturated brine, the density difference in
the Grashof number is very small. In this situation the mass transfer mechanism
could be significantly different from that which applies to unsaturated brines. A
separate set of experiments was planned to examine the behaviour of ore in contact
with a saturated brine. In these experiments, weight losses as a function of time were
to be monitored to examine whether or not dissolution occurred, and the relative
importanceof the 'blinding' phenomenon.
222
Dissolution of Potash Ore in Solutwns of High Sodium Chloride Concentration
Experimental Procedures
(i) Density and Viscosity measurements
Densities and viscosities as functions of solution compositions were determined
experimentally for the KC1-NaC1-water system. The experiments were performed at
25°C and 29°C. The temperature of the entire test chamber was held constant (at
either 25°C or 29°C) to eliminate temperature differences. Brine samples were
prepared by dissolving accurately weighed amounts of reagent-grade KCl and NaCl
in distilled water. Prior to testing, the brine samples were allowed to stand in sealed
containers for at least 16 hours after preparation, to ensure that the temperature had
equilibrated. The densities were measured by determining the weight and volume of
the brine samples using 100 ml volumetric flasks. Capillary tube viscometers were
used to measure the viscosities of the brine samples. Blank determinations were
performed using distilled water to verify the flask volumes and the calibrationsof the
capillary tube viscometers.
(i) Ore Sample Preparation
The dissolution-rate tests were performed on relatively small laboratory specimens
of potash ore. Most of the tests were conducted on reconstituted ore samples
prepared by combining measured mass fractions of KCl crystals and NaCl crystals.
The crystals were obtained from the dense media flotation circuit at the International
Minerals and Chemical Corporation plant in Esterhazy, Saskatchewan, and their
purities ranged between 95% and 98%. They were screened to provide come (A
mesh, ds0 = 6 mm) and medium sized (-4+8 mesh, ds, = 3 mm) fractions. The fine
crystal fraction consisted entirely of -8 mesh material. The medium-sized crystals
were used for the majority of the tests. The coarse and fine size fractions were used
for one series of tests designed to examine the effects of crystal size.
The crystal mixtures were pressed using a stainless steel mould and a 50 ton
hydraulic press. The reconstituted ore specimens were formed by applying a
pressure of 33 Mpa over a 24 hour period. For most tests, the pressed ore samples
were cut and trimmed to nominal dimensions of 12 cm x 10 cm x 2 cm thick. One
12 cm x 10 cm face was available for dissolution and all the other surfaces were
sealed with a moisture resistant epoxy sealant. The exposed face was sanded to a
smooth finish.
Some samples of raw potash ore (48% KCI, 49% NaC1) were also used during
the experiments. These ore samples were cut from 200 mm diameter core samples
provided by Cominco Ltd. The raw ore samples were prepared in the same manner
as the artificial samples so that all surfaces except one were sealed, and the exposed
surface was sanded to a smooth finish.
(ui) Data Acquisition System
Brine temperatures were measured using type-T thermocouples. Immersed
specimen weights were measured using 0 to 500 g electronic load cells supplied by
Cell Builders Inc. The load cells were temperature compensated over the range 0 to
65°C and their precision estimated as 0.1 g in isothermal conditions. A Seimecric
Model M8082A analog-to-digital converter and a Compaq XT personal computer
were used to monitor and record the instrument readings. Most of the tests were
conducted at 29"C, with a final set of measurementsat 25,30 and 35°C.
223
R.G. Gillies,D N . Madge and C A . Shook
(iv) Short-Duration Tests
Tests were performed to determine weight-loss rates for reconstituted potash ore
specimens formed using medium sized crystaIs. AU the tests were conducted at
29°C. Four grades of ore (0,40,60 and 100%KCl) were tested. Samples of raw ore
were also tested. The samples were suspended in 4 litre brine samples which
contained 7% KCl (by mass) and various levels of NaCl. The NaCl concentrations
were 15, 18.21 and 22.7% by mass. The latter brine sample was saturated with
respect to NaCl.
The actual weight-loss rates were determined using the measured immersed
weight of the specimen, the brine density (p) and the solids (ore)density @s> using
the relationship:
Actual Weight = Immersed Weight / [l - @/ps)]
Most of the weight losses were measured for approximately 30 minutes. For the
tests with the saturated brines, &he measurements were continued for a longer period
of time (nominally 20 hours). A layer of mineral oil was placed over each brine
sample to prevent evaporation during the 20-hour long tests.
During the test, some dispersed solids collected at the bottom of the beakers.
Some of this sediment was insoluble material released from the ore block during
dissolution, and some consisted of salt crystals which had become detached from the
ore block during the dissolution process. The solids were collected at the end of each
test by decanting and filtering the brine samples. The solids were submitted to the
Pilot Plant Laboratory of the Potash Corporation of Saskatchewanfor analysis.
The tests were initiated by lowering the ore slab gently into the beaker containing
the brine solution. Any air trapped within the slab escaped in the first two or three
minutes. This effect, and the fact that the slabs could not be lowered in a completely
reproducible manner, required the data for the first few minutes to be rejected.
Thereafter, the weight-loss rates in the short-term tests were stable with respect to
time.
The test conditions and weight loss results are summarized in Table 1. Of
particular interest is the fact that eddies were not observed near the surface of the
slabs when the most concentrated solutions were used. The solution compositions
are shown as points D, E, F and G in Figure 2.
(v) Long-Duration Tests with Saturated Brine
Longduration tests were performed to determine weight-loss rates for potash ore
specimens formed using coarse, medium and fine crystals. In this case, the
composition was 40% KCl and 60% NaCl. A raw ore sample was also used. The
specimens were nominally 12 cm x 10 cm x 5 cm thick. The back and the two sides
of each specimen were sealed with moisture resistant epoxy so that the top, bottom
and one vertical face were available for dissolution.
The tests were canied out using brine which contained 7% KCl and 22.7% NaCl
(saturated with respect to NaC1). Each specimen was immersed in a relatively large
tank of brine (48 lilres) to minimize the change in brine concentration during the test
The test temperature of 29°C was maintained by regulating the temperature of the
224
Dissolutionof Potash Ore in Solutwm of High Sodium Chloride Concematwn
Table I . Summary of test conditionsfor short-duration experiments
(temperature = 29°C; medium sized crystals).
lo0
7.0
15.0
0.5
150
100
7.0
18.0
0.5
70
A B
loo
7.0
21.0
0.5
29
C
100
7.0
22.7
20.5
3.1
C
60
7.0
15.0
0.5
270
&B
60
7.0
18.0
0.5
98
A. B
60
7.0
21.0
0.5
'34
C
60
7.0
22.7
205
1.6
C
40
7.0
15.0
0.5
250
AB
40
7.0
18.0
0.5
120
AB
40
7.0
21.0
0.5
14
C
40
7.0
22.7
20.5
0.7
C
0
7.0
15.0
0.5
170
A. B
0
7.0
18.0
0.5
34
A, B
0
7.0
21.0
0.5
9
C
0
7.0
22.7
20.5
0
E
Raw on
7.0
15.0
0.5
I10
A B. D
Raw on
7.0
15.0
0.5
170
A B. D
Raw olt
7.0
18.0
0.5
81
A, B. D
Raw OIC
7.0
21.o
0.5
26
C, D
Raw ore
7.0
22.7
0.5
0
E
A.
B
Key for Observations:
A.
B.
C.
D.
E.
Rapid dissolution o c c u d and density cumnt eddies were visible to the naked eye.
Significant roughening of the surface occurred during thc test
A small amount of surface rougbcning occurrrd during thc test
A film of insoluble mated colleaed at the dissolving face.
The om specimen did not change noticeably during the test.
room. Layers of mineral oil were placed above the brine samples to prevent
evaporation from the tanks.
Dispersed solids were collected from the bottom of the tanks and submitted for
analysis. The test conditions and dispersed solids (sediment) analysis are
summarized in Table 2.
225
R.G. Gillies.DN.Madge and CA. Shook
Table 2. Summary of test conditions for long-duration experiments designed to
examine the effects of ore crystal size (T = 29°C).
On
Quality
crystal
Jize
Lengthof
Observations
BrincComposition(mus)
% KCI
% NaCl
(See key following
Table 1)
Test-(hrs)
% KCI
coarse
40
732
7.0
22.7
B
Medium
40
1176
7.0
22.7
B
Fine
40
732
7.0
22.7
B
-48
I224
7.0
22.7
B, D
ROW
on?
Table 3. Swnmary of test conditwm for long-duration tests to examine the effects of
brine quality and temperature (ore quality = 40% KCl ;medium sized crystals).
2s
8.0
22.2
237
1.8
4.9
15.8
19.3
B
25
9.0
21.6
237
I .?
4.6
82.1
13.3
B
25
10.0
21.1
237
2.2
17.8
10.6
11.6
C
30
8.0
22.2
311
I .4
2.3
915
6.5
B
30
9.0
21.7
311
1.4
3.2
863
11.3
30
10.0
21.2
311
2.4
s.1
43.4
22.4
B
B
35
8.0
223
160
1 .S
6.6
88.1
5.5
B
35
9.0
21.1
160
3.1
2.2
91.1
4.6
B
35
10.0
21.2
160
3.4
113
l3.6
12.6
C
(vi) Long-Duration Tests for Temperature and Brine Composition Effects
Three series of long duration tests were performed to examine the effects of brine
composition and temperature on the potash dissolution process for saturated brines.
The tests were conducted at temperatures of 25.30 and 35OC using brines with KCl
concentrations of 8,9 and 10%by mass. In each case, the brines were saturated with
respect to NaC1.
The brine samples were contained in 4000 ml beakers which were placed in a
large temperature controlled water bath. A layer of mineral oil was placed over each
brine sample to eliminate evaporation.
The ore specimens contained 40% KC1 and 60% NaCl and were prepared by
pressing medium sized crystals. The specimens were trimmed to nominal
dimensions of 12 cm x 10 cm x 2 cm thick. One 12 cm and 10 cm face was exposed
to the brine and all other faces were sealed with epoxy. The exposed face was
sanded to a smooth finish.
226
Dissolutim of Potash Ore in Solutions $High Sodium Chloride Concentration
The tests were conducted for several days to establish long term trends. The test
conditions and analysis of dispersed solids material are summarized in Table 3.
Experimental Results and Discussion
(a) Viscosities and Densities of Solutions of KCl and NaCl
The relative Viscosities (dynamic viscosity divided by the viscosity of water at the
same temperature) became more sensitive to KCl content as the NaCl concenmtion
of the solution increased. Densities of the solutions were primarily functions of the
total solute molarity. Data for these solutions can be obtained from the authors.
(b) Unsaturated-Brine Dissolution Rates
The mass-transfer rate equation (Equation 3) is known to be appropriate when a
single component (pure KCl or NaCl) dissolves. In Figure (2), for a solution with
bulk composition F (x,J. the surface concentration (xb) is given by the interseCtion
of line FH with AB, or FJ with BC. In the absence. of surface mction effects, the
mass transfer coefficients should be consistent with those obtained in previous friee
convection experiments.
If Equation (3) is to be used to calculate mass transfer rates when two
components dissolve, then the mass transfer coefficient (kx) must be defmed. In
principle, k, should be different for the two species because, on an area-averaged
basis, there are two concentration boundary layers (one for each solute), although
there is only one velocity boundary layer. However, the diffusivities of NaCl and
KC1 differ by less than 25% so that in this particular case a simpler approach could
be used. In converting experimental dissolution rates to mass transfer coefficients
and Sherwood numbers, Equation (3) is applied as if the ore were a pseudohomogeneous substance. Specifically, when two solutes are present:
a) Nl is the total dissolution flux (lcg/m2s);
b) xl= is the sum of the mass fractions of the two salts in the bulk of the solution;
c) xls is the total solute mass fraction at the surface, computed for point B. This is
an alternative to the method of Oszahin and Husband (1967). Using the ores
containing 40% and 60% KCl, and a solution with composition F, the method of
Husband and Oszahin predicts the surface compositions determined by the
intersection of line FK with AB, or F'L, with BC.
When Sherwood numbers (Sh) or Schmidt numbers (Sc) were calculated, mean
diffusivities for the solution were computed from the diffusivities of pure NaCl and
pure KCl in water assuming a linear mixing rule. Values of these diffusivities at
25°C are given by Harned and Owen (1958) and were corrected for the effect of
temperature using the activation energies quoted by Simon (1981). Viscosities and
densities were interpolated from the experimental values. Unless noted otherwise,
diffusivities, Viscosities and densities were determined from the arithmetic mean of
the surface and the bulk concentration.
These definitionsobviously involve additional approximationsin two-component
dissolution. Comparison of the two-component results with single-component
dissolution should help to assess these simplifyingassumptions and definitions.
227
R.G. Gillies, D.N. Madge and CA. Shook
(c) Single-ComponentDissolution by Unsaturated Solutions
The experimental N, values were used to obtain k, values, which are shown in
Figure 3 as values of the Sherwood number plotted against the Raylei& number. At
the higher solution concentrations (points E and F in Figure 2), where the lower
Rayleigh numbers occur, the experimental values are close to those predicted by the
laminar boundary layer equation. However, for the brine with the lowest
concentration (point D in Figure 2), the S h e r w d numbers are significantly higher,
so that the measurements are closer to values predicted by the correlation of
Churchill and Chu. Considering the observations shown in Table 1, it seems likely
that the transition from laminar to turbulent flow plays an important role for
specimens of this length. This probably explains the relatively poor reproducibility
(30% deviations) between replicate tests despite the precision of the weight loss
measurements. This relatively poor reproducibility for small specimens was noted
previously by Husband (1969).
(d) Two-Component Dissolution by Unsaturated Brines
These rates were expressed as k, values using Equation (5) and the assumptions
stated previously. These results are also shown as values of the Sherwood number
plotted against the Rayleigh number in Figure 3. The invariant point (point B in
Figure 2) was used to calculate the equilibrium concentration (xis) in this case. The
agreement with the singlecomponentSherwood numbers is satisfactory. The points
shown at Rayleigh numbers near l O I 3 are the dissolution rate measurements of
Husband and Oszahin (1967) interpreted in the same way as the mixture data
obtained in this investigation. In calculating the Sherwood and Schmidt numbers for
the measurements of Husband and Oszahin, the effects of temperature on the
physical properties were also calculated using the activation energies quoted by
Simon (1981). Husband and Oszahin's inference that their dissolution process was
laminar remains valid when their data are interpreted using the invariant point as the
surface condition. This similarity in the values of the mass transfer coefficients
calculated from two different definitions of the interface condition occurs because
they used water as the dissolving fluid.
If the method of Husband and Oszahin were used to determine the concentration
xls forthe brines used in the present investigation, the mass transfer coefficients and
Sherwood numbers for the mixtures would be significantly higher than those which
apply for the pure salts. These higher coefficients occur because the concentration
difference between the solution and point B in Figure 2, is greater than that between
the point and the saturation lines AB and BC along lines such as FK or FL. It
therefore appeats that the invariant point composition is useful for estimating xIsr
when the procedure described previously is employed.
Although the range of Rayleigh number is rather restricted, the evidence suggests
that the laminar boundary layer and Churchill-Chu correlation provide useful
indications of the magnitudes of free convective mass transfer coefficients for ores
containing 40 to 60% KCI. The experimental Sherwood numbers are actually
somewhat higher than the limits implied by these correlations. These deviations
could be due to effects which are neglected in a pseudo-single component approach
and which are shown in Equations (5)and (8).
228
Dissolution of Potash Ore in Solutwns @High Sodium Chloride Concematwn
10000
5
v, 1000
1
Figure 3. Experimental Sherwood numbers as functions of Rayleigh number for
dissolution of pure KCl, pure NaCl and mixtures of KCl and NaCl in brines D , E
and F (see Figure 2). (Results of Husband and Oszahin are also shown),
10000
Raw Ore
100
:
I
1111111
1
I11111
I
I
l m l
I
I Ild
229
R.G. Gillies,DN.Madge and CA. Shook
The experimental measurements of Husband and Oszahin (1967) are of
considerable importance in establishing the mechanism of the dissolution. Their
temperature range (21.1OC to 54.4OC) was relatively wide, and the modest change in
rate which they observed shows that a surface reaction of the type detected by Simon
(1981) does not play an important role in two species free convection dissolution.
(e) Dissolution by Saturated Brines
The short-durationtest results in Table 1 show that rates of dissolution by saturated
NaCl solutions were very low. Even when the test period duration was increased to
20.5 hours, the changes in weight were probably too small for rates to be measured
reliably (the pure NaCl slab slightly increased in weight). These unsatisfactory
results led to the longduration tests which are summarizedin Table 2.
The first set of longduration tests examined the effect of sample heterogeneity
by using reconstituted ores with different crystal sizes and a sample of raw ore.
Although there was some difference between the results, the rates were similar and
show that blinding did not cause dissolution to cease. The insolubles in the raw ore,
which led to obvious surface deposits, only seemed to reduce the dissolution rate by
about 50%. The rates gradually decreased with time.
Because three surfaces were exposed to the brine in these tests, it was difficult to
draw firm conclusions about the cause of the gradual decrease in the rate with time.
However, it seems probable that it was due to the KCl content of the slab becoming
exhausted. A further series of tests un&r conaolled hydrodynamic conditions was
undertaken, with the contact time somewhat reduced and the KCl concentration
increased.
In the second set of saturated-brine dissolution tests shown in Table 3, the
specimens were vertical so that the orientation was identical to that used in the
unsaturated-brine dissolution measurements. In these experiments, there was an
initial period of 25 to 100 hours in which the dissolution rates were relatively high,
i.e. consistent with free convective dissolution under conditions of low Grashof
number. Thereafter, dissolution appeared to continue at a very low but apparently
constant rate. The variable initial rates are understandable considering that the brine
compositions were prepared at saturation compositions determined from
interpolation of the phase diagram (Linke, 1965), and that the slabs released small
quantities of air when fmt immersed in the solutions.
In view of their possible relevance to solution mining, the low ultimate rates were
estimated by ignoring the initial periods. To interpret these rates, the mass transfer
pracess in a brine saturated with respect to NaCl was assumed to be:
NaCl(,)
+ KCl(,) = NaC4,) + KC+,,)
(9)
Using rate equations to describe transfer between the saturated surface (s) and the
bulk (m) of the solution, and omitting the subscript x from the mass transfer
coefficients:
230
Dissolution of Potash Ore in Solutwns @High Sodium chloride Concematwn
the net rate of mass decrease is given by (NKAK - NNaANa), where AK and ANa
denote the areas at which the transfer takes place. Thus:
where
The term in the square brackets in Equation (11) is the difference between the
mass fractions at the invariant point (s) and that in the bulk of the saturated solution
& is the effective transfer coefficient.
Values of k,were calculated from the experimental weight-loss rates (lcg/m2 s).
They are very low compared to the free convective mass transfer coefficients in
unsaturated brine, presumably because dissolution actually takes place at a very
small fraction of the surface area A. However, like the rates themselves, these
saturated-brine mass transfer coefficients are strongly temperature dependent.
Figure 5 is an hhenius plot of the k,values; the activation energy is of the order of
14 kcallmole, which is much higher than that associated with a diffusion mechanism.
The activation energies observed by Simon (1981) for pure KCl and pure NaCl
dissolution surface reactions were lower than that implied by Equation (9). It should
be noted that Simon used an equation similar to Equation (10) to define the rate
constant for the reaction process.
(-);
1/T ( K-’)
Figure 5. Dissolution rate constants (k,,,)for NaCl-saturated brines versus 1IT.
To interpret the low rates of mass transfer in the presence of a saturated NaCl
solution, we must reconsider the weight loss results. A slab of initial mass
approximately 1.2 kg, containing approximately 480 g KC1, lost approximately200g
as the KCl was removed and NaCl was deposited. During this time the surface
roughened and the mass transfer process was not dominated by free convection in
231
R.G. Gillies,DN.Madge and CA. S h k
the solution. Therefore, it Seems likely that the dissolution ~ ~ O C proceeds
~ S S
within
the slab rather than at the surface. According to this view, the solution must penetrate
the slab, presumably at crystal junctions, dissolve KC1 and deposit NaCl in
accordance with Equation (9). A solution having composition near that of the
invariant point must return to mix with the bulk of the solution. Because of
differences in the rates of dissolution and the specific volumes of the two solid
species, the porosity of the slab increases with time. This porosity would appear as
an increase in roughness. Hence, the low k,,, values are due in part to the relatively
small areas at which simultaneous dissolution and deposition take place within the
slab (per unit area of slab surface). However, the process must be complex because
increasing the contact area between KC1 and NaCl (by decreasing the crystal sizes)
did not increase the overall rate.
Conclusions and Recommendations
The experimentssuggest that:
1) Free convective dissolution of ores in unsaturated brines occurs at rates
comparable to those predicted by accepted correlations, if the interfacial
compositionis assumed to be that of the invariant point.
2) Ore containing 40% KCl dissolves in saturated brine at rates which are relatively
low, after an initial phase lasting up to 100 hours. These rates do not decline
significantly with time. The dissolution process in this case appears to be
controlled by a reaction, rather than diffusion.
3) Although the insolubles cause dissolution rates to be somewhat lower for raw ore
than for reconstituted ore, the effect is only moderate. Since the cost of preparing
reconstituted ore slabs is relatively high, further experiments with reconstituted
ore are probably not justified.
The physical process by which dissolution into a saturated solution occurs seems
to be worth investigation. It is our view that the saturated solution must penetrate the
slab and invariant concentrationbrine must leave it simultaneously.
Although use of the invariant point composition seems to allow mass transfer
rates to be estimated for ores containing between 40 and 60% KCI, it is not clear if
this assumption would be justified for other compositions and it is obviously not
appropriate for the pure salts. The behaviour of lean ores containing between 0 and
40% KCI, and rich ores containing between 60 and 100%KC1, should be examined
with respect to Conclusion 1 above.
Acknowledgments
"his investigation formed part of a study conducted by Kilbom Western Inc. for the
Saskatchewan Potash Producers Association. Mr. P. Schergevitch of the
Saskatchewan Research Council conducted the experiments. The authors are
indebted to the SPPA for permission to publish the results. Helpful discussions with
the SPPA management committee, Messs. A. Banks and D. Thompson of Kilbom
and Dr.W.H.W. Husband are acknowledged.
232
Dissolution of Potash Ore in Solutwm @High Sodium Chloride Concentration
Nomenclature
A
k e a of lab surface CL2,
C
Concentration(ML")
D
Diffusivity (L2T1)
Acceleration due to gravity (LT-9
g
h
Heat transfer coefficient m3)
FIUX relative to mean velocity (ML-~T')
J
Effective mass transfer coefficient (LT')
k,
Is,
Mass transfer coefficient (LT-')
Vertical dimension of dissolving slab Q
L
m
Mass of slab 0
N
Mass flux relative to fvred coordinates (ML'2T-1)
t
Time 0
X
Mass fraction solute
Distance perpendicular to surface (L)
Y
P
viscosity (ML-IT')
P
Density (ML-3)
Subscripts
av
Mean value of solutions
S
Surface value (invariant composition assumed)
X
Implies concentrationexpressed as mass fraction
0
Bulk solution value
References
Churchill, S.W. and Chu. H.H.S. 1975. Correlating Equations for Lammar and Turbulent Free
Convection from a Vertical plate. IN. J.Hear and Mass Tramfa, 18.1323-1329.
Fujita, H. and Costing. L.J. 1960. A New Procedure for Calculating the Four Diffusion
Coefficients of Three Component Systems from Gouy Diffusiometer Data. J . Phys.
Chem., 64.1256-1263.
Ham& H.S. and Owen,B.B. 1958. The Physical Chemistry of Electrolytic Solutions, 3rd Ed.
American Chemical SOC.Monograph Series. Reinhold, New York.
Husband, W.H.W. 1969. Free Convection Mass Transfer from Vertical Surfaces. P h 9 .
Thesis in Chemical Engineering, University of Saskatchewan.
Husband,W.H.W. and Oszahin, S. 1967. Rates of Dissolution of Potash Ore. Can. J . Chem.
Eng.. 4,234-237.
Husband, W.H.W. and Shook, C.A. 1970. An Experimental Study of Free Convection Mass
Transfa h m High Vertical Surfaces to Liquids. Proc. 3rd Synpsium on Sail, Northem
Ohio Geological Society, pp. 360-370.
Linke, W.F. (Ed.). 1965. Solubilities of InorgMic and MetatOrganic Compounds, Vol. II,
4th Editwn. hexican Chemical Society, Washington, D.C.
Simon,B. 1981. Dissolution Rates of NaCl and KCl in Aqueous Solution. J. C r y d Growrh,
12.789-794.
Taylor. J.B.. Hunt, M.R., Despault, G.J. and Agyako, A.H. 1967. Selective Extraction of
Potassium Chloride fiom Saskatchewan Sylvinite Ore. Can. J. Chem. Eng.. 4.105-108.
Received 22 July 1993; Accepted after revision: 2 May 1994.
233
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